The "luminous capacitor", invented in 1936 by G. Destriau in Marseille, is known to consist (in its later, developed form) of a front glass plate covered with conducting, transparent oxide such as tin oxide or indium oxide, a resinous or vitreous insulator layer into which is embedded suitably-prepared electroluminescent (EL) zinc sulfide powder (basically ZnS supersaturated with copper), and a metallic rear electrode plate. Light is emitted if a high, audio-frequency electric field is applied.
This effect was developed to the state of practical applicability in the USA up to the years 1963-1964 but was then dropped because these luminous panels did not immediately satisfy all expectations. Besides, at that time the light-emitting diodes (LEDs) were coming up and diverted public attention.
The physical mechanism of light generation in embedded ZnS particles in high fields, as explained by A. G. Fischer in 1962, consists of alternate bipolar carrier injection from conducting copper precipitates into the surrounding luminescent ZnS host lattice, and radiative recombination at each field reversal.
One reason why these luminous panels were abandoned was that in brightly-lit rooms the visual contrast of EL information displays was poor, i.e., addressed elements of a display were hard to distinguish from unaddressed ones. This is because the body color of these panels is near-white, owing to the high refractive index (n = 2.4) of the white ZnS powder in its binder (like white paint). Therefore, the panel reflects much of the ambient light into the eye of the observer. For good readability the contrast of a display (the brightness quotient between an addressed segment and its background) should be at least as high as 6-fold. This means that the addressed elements should send 6 times as much light into the eye of the observer as is reflected there from the ambient illumination by the background of the addressed segment. This requires a high luminous intensity of the display segment, but high intensity is indeed difficult to obtain from these EL layers.